HK1158850B - A method of data rate adaptation for multicast communication - Google Patents

Description

Method of data rate adaptation for multicast communication

Cross Reference to Related Applications

The present application claims priority from an application EP 08305789.3 filed on 7.11.2008, entitled "A method Data Rate Adaptation For Multi case Communication", which is incorporated herein in its entirety.

Technical Field

The present invention relates to a data transmission technology, and in particular, to a method of adjusting a data transmission rate in a cable multicast system.

Background

There are existing specifications that define the communication and operational support interface requirements for systems that communicate data over cables. One of these specifications is the service interface specification (DOCSIS) for data over cable, an international standard that allows the addition of high-speed data transfer to existing cable television (CATV) systems, and is adopted by many cable operators to provide internet access over their existing hybrid fiber coax infrastructure.

Cable modems based on solutions such as DOCSIS are expensive and are not suitable for providing quality of service (QoS) in cable networks that is sensitive to real-time audio communication and video streaming services. It would be desirable to develop a new system for transmitting data over a CATV cable access network that can ensure good quality of service (QoS) at a reasonable cost and balance with existing standard protocols in a cable data system and a user terminal device or modem, such as a device on a local area network, that operates on an existing CATV system and provides internet-type data services for client devices. In such a system multicasting to user terminals would be an efficient way of distributing content to a multitude of user terminals that are to receive the same data. In multicasting, the transmission data rate is typically fixed to the slowest reception rate of the members in the multicast group. However, the slowest rate user terminal may change over time. The method of assigning the slowest fixed data rate user terminal to the data rate transmission in the multicast scheme cannot adapt to the change of user terminals within the multicast group. Another factor is the access point. If the access point increases transmission power to some stations, the increased power may allow for higher data rates to selected stations. A more efficient and adaptable data rate determination mechanism would be beneficial for the reservation of bandwidth in a cable system.

Disclosure of Invention

According to an aspect of the invention, a method by an access point of adaptively adjusting a data rate of a multicast transmission, comprises: a multicast message frame is transmitted from an access point to a multicast group of stations at a first data rate. The access point then receives an acknowledgement of successful frame reception from the station using the first data rate. The first data rate is then increased to a second data rate for a temporary time period. During the temporary time period, multicast message frames are transmitted to the stations and multicast frame loss is evaluated while using the second data rate for multicast transmissions. The evaluation includes comparing the frame loss to a threshold. If the frame loss reaches the threshold, the frame loss is too high and the access point reduces the second data rate to the first data rate. However, if the frame loss is less than the threshold, the second data rate is used for the new data rate for the multicast message/data/frame.

As one aspect of the invention, adaptive data rate adjustment may be used in cable systems utilizing IEEE802.11 frames in a time division multiplexed protocol. Changing the rate from the first data rate to a second, higher data rate is based on feedback from stations receiving the multicast message. The increase in data rate may be caused by an increase in received signal strength of the reference station in the multicast group having the lowest signal strength. Another way to increase the data rate is to receive a plurality of consecutive reply signals from the reference station. The increased data rate is then tested over a period of time to determine whether a higher data rate should be employed as the new multicast data rate.

In another aspect of the invention, a station actively participates in multicast data rate adjustment by first receiving a beacon frame with a minimum signal strength value for a reference station within a multicast group. The station measures a new received signal strength value and determines whether the new received signal strength value is less than a minimum signal strength value from the beacon frame. The station transmits the strength value of the new received signal to the access point if the strength value of the new received signal is less than the minimum signal strength value. The station determines whether it is a reference station having a minimum signal strength value acquired from the beacon frame information. Then, if the first station is a reference station and the strength value of the new received signal is greater than the minimum signal strength value, the station transmits the strength value of the new received signal to the access point. The access point may then perform the complementary parts of the present invention.

Further features and advantages of the invention are apparent from the following detailed description of illustrative embodiments thereof, which proceeds with reference to the accompanying drawings.

Drawings

Fig. 1 illustrates a simplified exemplary TDF access network architecture;

FIG. 2 illustrates the 802.11MAC sub-layer in the OSI reference model;

fig. 3 illustrates an implementation of the TDF transmission entity in the OSI reference model;

FIG. 4 illustrates an example of a mode entry process;

fig. 5 illustrates an example of a TDF superframe structure;

fig. 6a illustrates an example Station (STA);

FIG. 6b illustrates an example Access Point (AP);

FIG. 7 illustrates an example information element format;

fig. 8 illustrates an example channel sensing beacon frame;

fig. 9 illustrates an example method for a Station (STA);

fig. 10 illustrates an example method for an Access Point (AP); and

fig. 11 illustrates an example method of adaptive data rate adjustment.

Detailed Description

General description

As used herein, "/" denotes alternative names for the same or similar components or structures. That is, "/" can be considered an "or" as used herein. Unicast transmissions are between a single sender/transmitter and a single receiver. A broadcast transmission is between a single transmitter/transmitter and all receivers within reception range of the transmitter. Multicast transmissions are between a single sender/transmitter and a subset of receivers within reception range of the transmitter, where the subset of receivers within reception range of the transmitter may be the entire set. That is, multicast may include broadcast and is thus a more general term than broadcast as used herein. Data/content is transmitted in packets or frames.

To provide data services over existing coaxial cable TV systems (CATV), at least one embodiment deploys a Time Division Function (TDF) protocol compliant Access Point (AP) and Station (STA) in a cable access network. The AP and the STA are connected via a coupler in a hierarchical tree structure. In this manner, a user may access a remote Internet Protocol (IP) core network at home via a cable access network. User access to the IP core enables such things as, but not limited to, internet access, digital telephone access (e.g., voice over internet protocol), and cable television programming. An example network architecture 100 is illustrated in fig. 1.

Fig. 1 depicts one embodiment of a network that accesses an IP core network 105. The IP core network may be any digital network using the internet protocol or an equivalent digital data transfer protocol. In the exemplary embodiment of fig. 1, a Time Division Function (TDF) protocol compliant Access Point (AP)110 has a network interface 116, such as a wired LAN or optical interface, to connect with IP core network 105 and a cable interface 112 to connect with a cable access network. Many such access points may be connected to the IP core network. The cable access interface 112 of the AP 110 may access any form of cable, such as fiber optic, coaxial, or other physically connected communication medium. As a result, the cable network may include any form of cable, such as fiber optic, coaxial, or other physically connected communications media. The wired network may include a signal coupler 115 and, if desired, a distribution medium such as interconnecting cables 117 and 119. Although fig. 1 shows only two such distribution cables, it is understood that numerous such distribution connections are possible.

In the example of fig. 1, distribution cables 117 and 119 are connected with TDF protocol compliant Stations (STAs) 120, 140 via cable interfaces 122 and 142, respectively. STA cable interfaces 122 and 142 may also access any form of cable, such as fiber optic, coaxial, or other physically connected communication media. STAs 120 and 140 are TDF compliant and act as user terminals that can connect with the cable access network with numerous interfaces for users/clients. Those interfaces include, but are not limited to, user/client devices operable on conventional physical Local Area Networks (LANs) and Wireless Local Area Networks (WLANs). One example LAN is an ethernet compatible network. One example wireless network is an IEEE802.11 compliant wireless network.

Fig. 1 depicts that station 1 modem 120 and station N modem 140 have similar interfaces. However, this representation is merely exemplary, as stations of different capabilities may be connected to the cable network, assuming that the stations are communicatively compatible with the AP 110. For example, the station modem may have all of the user interfaces shown in fig. 1 or only a selected subset. In fig. 1, station 1 is configured to support LAN interface 124, LAN interface 124 driving LAN connection 121 to a physically wired LAN 130 having branches (stubs) 130a, 130b, and 130 c. The branch supports LAN compatible devices such as set top boxes 132 for television or other services, Personal Computers (PCs) 134 for network services such as internet services, and other LAN compatible devices 136, which may include devices that support digital services for providing any type of multimedia services such as video, audio, telephony, and data. Such LAN compatible devices 136 include, but are not limited to, facsimile machines, printers, digital telephones, servers, and the like. Fig. 1 depicts that station 120 also drives antenna 125 via WLAN Radio Frequency (RF) port 126 to provide wireless services. The resulting transmission 123 may be used by the WLAN-compatible device 138 to provide services including any multimedia voice, audio, telephony, and data to the user/client. Although only one wireless device 138 is shown, a multitude of such wireless devices may be used. Similarly, station N also includes LAN interface 144 to drive LAN connection 141 for physical connection 150 having branches 150a, 150b, and 150 c. Such a branch supports communication with such LAN compatible devices as set top boxes 152, PCs 154, and other devices 156. WLAN RF port 146 supports antenna 145 to provide link 143 for communication with WLAN device 158. Those skilled in the art will appreciate that appropriate interfaces exist for each of the network interface 116, cable interfaces 112, 122, 142, wired LAN interfaces 124, 144, and WLAN RF interfaces 126, 146 in fig. 1.

In one embodiment of the network 100, both TDF compliant APs and STAs implement protocol stacks separately in the logically linked control sublayer, MAC sublayer, and physical layer according to the IEEE802.11 family of specifications. However, in the MAC sublayer, the TDF AP and STAs replace the IEEE802.11 frame transmission entity with a TDF frame/message/signaling entity. Thus, the MAC sublayer of TDF APs and STAs includes IEEE802.11 frame encapsulation/decapsulation entities and TDF frame transmission entities, while the MAC sublayer of IEEE802.11 compliant APs and STAs includes IEEE802.11 compliant frame encapsulation/decapsulation entities and IEEE802.11 frame transmission entities. For integrated APs and STAs, the TDF frame transmission entity and the IEEE802.11 frame transmission entity may coexist at the same time in order to provide both IEEE802.11 and TDF functionality. Switching between the two modes may be achieved by manual or dynamic configuration.

An example system such as that in fig. 1 that includes one or more APs, a cable network, and one or more STAs may also be referred to as an asymmetric data over coaxial (ADoC) system. One or more protocol compliant ADoC Access Points (APs) and one or more Stations (STAs) are deployed in an ADoC cable access network. Thus, as used herein, the terms "ADoC system" and "TDF system" are considered interchangeable, as ADoC system is a particular implementation of TDF system. In the ADoC system as in fig. 1, the AP and the STA are connected via a coupler in a hierarchical tree structure of a cable network including typical elements configured as a cable network, such as a cable, a splitter, an amplifier, a repeater, a switch, a converter, and the like.

TDF protocol introduction

AP 110 and STAs 120 and 140 of fig. 1 employ the TDF protocol for communication over the cable medium. In one embodiment of the TDF protocol, IEEE802.11 frames are transmitted over the cable medium rather than over the air. The IEEE802.11 mechanism is employed with the goal of utilizing the mature hardware and software implementation of the IEEE802.11 protocol stack. Thus, TDF using IEEE802.11 frames is used in the cable network of fig. 1 as a communication standard between an AP and its associated STAs.

One feature of TDF is its unique medium access control method for transmitting IEEE802.11 data frames. In one embodiment, the tdtdfdf does not employ the conventional IEEE802.11 DCF (distributed coordination function) or PCF (point coordination function) mechanism to exchange MAC frames including MSDUs (MAC service data units) and MMPDUs (MAC management protocol data units). Instead, the TDF uses a time division access method to transmit MAC frames/messages/signals. TDF is thus an access method that defines the implementation of the frame transport entity located in the MAC sublayer.

Fig. 2 illustrates the standard IEEE802.11 MAC sublayer protocol in the Open Systems Interconnection (OSI) reference model. By way of comparison, fig. 3 illustrates details of the frame transmission entity for the TDF protocol in the OSI reference model.

In one embodiment, stations such as STA 120 and STA 140 operate in two communication modes. One mode is the standard IEEE802.11 mode of operation, also referred to as the user/client interface mode, which follows the frame infrastructure and transport mechanisms defined by the IEEE802.11 family of standards. The other mode is the TDF run mode, which is also referred to as the cable interface mode. In one embodiment, the determination of which operating mode to enter when the STA starts up is indicated in fig. 4. Once the STA receives the synchronization frame/message/signal from the AP, the STA enters the TDF mode. If no sync frame is received within a preset timeout, the STA remains or switches to user/client interface mode. More operating mode switching criteria are provided herein.

TDF protocol function description:

access method

The physical layer in the TDF station may have multiple data transmission rate capabilities to allow for the implementation of dynamic rate switching for the purpose of improved performance and device maintenance. In one embodiment, a station may support a multitude of data rates. The data rate may be statically configured before the TDF station enters the TDF communication procedure and remains the same throughout the communication process. On the other hand, the TDF station may also support dynamic data rate switching during service. The criteria for data rate switching may be based on channel signal quality, among other factors. The present invention is directed to dynamic data rate switching during multicast transmission.

The basic access method of the TDF protocol is Time Division Multiple Access (TDMA), which divides the same channel into multiple different time slots to allow multiple users to share the channel. The STAs receive the downloads and transmit the uploads in rapid succession, one after the other, each using their own time slot within the TDF superframe specified by the AP. Downlink traffic is defined as data transmission from the AP to the STA. Examples of downlink traffic include requested digital data/content, such as audio or video requested by a user/client device. The downlink data may be unicast, broadcast, or multicast. Uplink traffic is defined as data transmission from the STA to the AP. Examples of uplink traffic include user requests for digital data/content or commands to the AP to perform some function. The uplink data may be unicast or multicast.

Fig. 5 illustrates an example of a TDF superframe structure and slot allocation of a typical TDF superframe when there are "n" STAs. As shown in fig. 5, there are a fixed number of time slots per TDF superframe, including one synchronization slot for transmitting clock synchronization information from the AP to one or more STAs and one contention slot for transmitting a registration request for uplink time slot allocation. The indicated registered STA uses the uplink time slot allocated to the STA to transmit data and some management frames to the AP. The AP uses the downlink slot to transmit data to the STA and to register a response management frame. All slots, except the synchronization slot, may have the same duration. The multicast period of fig. 5 uses one or more predetermined number of time slots to accommodate the length of the multicast data. The value of the typical slot duration is defined to allow transmission of at least one maximum IEEE802.11 PLCP (physical layer convergence Protocol) Protocol Data Unit (PPDU) in one normal slot for the highest data rate mode. The duration of the synchronization slot may be shorter than the duration of a typical slot because the clock synchronization frame transmitted from the AP to the STA in this slot may be shorter than the IEEE802.11 data frame. The TDF superframe of fig. 5 is an example of a format in which the slot fields are ordered into synchronization slot, contention slot, multicast period, downlink and uplink slot pairs. Other orderings of the slot fields are possible, provided that the synchronization slot occurs first in the superframe. For example, the following ordering is also possible: (i) synchronization time slot, downlink time slot, uplink time slot, contention time slot, (ii) synchronization time slot, uplink time slot, downlink time slot, contention time slot, and (iii) synchronization time slot, contention time slot, downlink time slot, uplink time slot. Other forms of organization are also possible.

In one implementation, a typical slot duration is approximately 300us, which is sufficient for a STA to transmit at least one maximum IEEE802.11 PPDU in one common slot of a 54Mbps mode. Each TDF superframe has a total of 62 time slots. In these slots, there are uplink and downlink slot pairs. When there are 20 STAs, each STA can be guaranteed access to the 680kbps uplink data rate. The downlink data rate depends on the presence of multicast data and the requirements for downlink data transmission at the AP. Finally, the duration of the superframe, which in one embodiment has a total duration of 61 typical slots and one synchronization slot, is approximately 18.6ms and may be defined to different values for different purposes. For example, if there is only one STA, it can be guaranteed that the STA has 4 slots in order to reach an uplink data rate of about 18Mbps and has a downlink data rate of 18Mbps (4 consecutive slots). In this manner, the duration value of the superframe is about 4 ms.

Frame/message/signal format

In the IEEE802.11 specification, there are three main frame types: data frames/messages/signals, control frames/messages/signals, and management frames/messages/signals. Data frames are used to exchange data from an access point to a station and vice versa. Depending on the network, there are several different kinds of data frames. The control frame is used together with the data frame to perform an area cleaning (area cleaning) operation, channel acquisition and carrier sense maintenance functions, and acknowledgement of received signals. Control frames work with data frames to reliably deliver data between access points and stations. More specifically, one important feature in exchanging frames is that there is a presence mechanism and there is an Acknowledgement (ACK) frame for each downlink unicast frame accordingly. This is present to reduce the probability of data loss due to unreliable wireless channels. The management frame performs a supervisory function. They are used to join and leave wireless networks and to move associations from access point to access point. As used herein, the term "frame" may also refer to a message or signal in all cases. Equivalently, the term "frame/message/signal" may also be used to denote equivalents.

In one embodiment of the TDF system, the STA passively waits for a synchronization frame/message/signal from the AP in order to identify the controlling AP. The synchronization frame is a frame of data located within the synchronization slot (slot 0) in fig. 5. Because the STA waits for the AP to transmit a synchronization frame, typical probe request and probe response frames found in wireless implementations of the IEEE802.11 standard are not needed. However, Acknowledgement (ACK) frames/messages/signals are used to ensure the reliability of data frame delivery.

In the TDF protocol, only some IEEE802.11 MSDUs and MMPDU types useful for data are used in the cable medium. For example, the data subtype in the data frame type is used to encapsulate upper layer data and for transmission between the access point and the station. New management frames may be used to accommodate clock synchronization requirements in the TDF system. For example, if additional information needs to be transmitted from the AP to the STAs, the additional information may be included in the synchronization frame.

In one embodiment of the TDF system, an IEEE802.11 beacon frame is periodically transmitted as part of the synchronization slot (slot 0) to synchronize STAs to the associated AP, as in the TDF superframe shown in fig. 5. A typical beacon frame is a management frame that includes a header and frame body information. In other words, the header includes the source and destination MAC addresses as well as other information about the communication process. The destination addresses may be set to all ones, which are broadcast Media Access Control (MAC) addresses. This instructs all STAs on the applicable channel to receive and process the beacon frame.

Access Point (AP) search and clock synchronization

The TDF protocol provides timing information distribution to all STA nodes. The STA listens to the synchronization frames/messages/signals in the synchronization slots of the superframe of fig. 5 to decide whether there is an active AP available. Once the STA enters TDF communications, the STA uses the synchronization frame to adjust its local timer based on which the STA decides whether it is its turn to send uplink frames. At any given time, the AP is the master and the STA is the slave during synchronization. If the STA does not receive a synchronization frame from the associated AP for a predetermined threshold period, the STA reacts as if the associated AP has stopped serving the STA. In this case, the STA stops communicating with the silent AP and starts looking for an active AP by listening to the synchronization frame again.

In TDF systems, all STAs associated with the same AP are synchronized to a common clock. The AP periodically transmits a specific frame called a synchronization frame containing AP clock information in order to synchronize the STAs in its local network. In one embodiment, the synchronization frame for transmission is generated once per TDF superframe time by the AP and sent in the synchronization slots of the TDF superframe.

Each STA maintains a local Timing Synchronization Function (TSF) timer to ensure that it is synchronized with the associated AP. After receiving the synchronization frame, the STA always accepts timing information within the frame. If the STA TSF timer is different from the timestamp in the synchronization frame received from the AP, the receiving STA sets its local timer according to the received timestamp value. Further, the STA may add a small offset to the received timing value in order to account for transceiver local processing.

STA registration with AP

Once the STA has acquired the timer synchronization information from the synchronization frame, the STA knows when slot 0 occurs. If the STA is not associated with any active AP, the STA attempts to register with the AP transmitting the synchronization frame. The STA associates with the AP by sending a registration request frame to the AP during the contention time slot, which is the second time slot in the TDF superframe of fig. 5. In one embodiment, the duration of the contention slot and the registration request frame/message/signal structure are designed to allow multiple registration request frames to be sent in one contention slot. Based on this design, the contention slot is divided into equal length sub-slots.

Once the STA detects an active target AP, the STA selects one sub-slot in the contention slot to send a registration request frame to the AP. The purpose of this action is to reduce the chance of collisions when many STAs start at the same time and attempt to register with the same AP at the same time. The registration request may occur according to the following method:

A. after allocating the uplink slot, the STA stores the number of allocated uplink slots. The allocated uplink time slot indicates the position of the time slot in the entire pool (pool) of uplink time slots.

B. Each time a STA requests an uplink slot, the AP allocates the same uplink slot to the same STA.

C. When it is time to select a slot in which to transmit a registration request frame, if there is a stored allocated slot value, the STA sets the number of subslots to the allocated value. If there is no such value, the STA randomly selects a sub-slot among the available slots. The STA then sends a registration request frame to the AP in a randomly selected sub-slot.

The STA lists all the data rates it supports at this time and also sends some information, such as the received signal to carrier/noise ratio, in the registration request frame. The STA may send several consecutive registration request frames with different data rates supported. After the outgoing frame, the STA listens for registration response frames/messages/signals from the AP.

Upon receiving the registration request frame from the STA, the AP sends back a registration response frame of a different kind to the STA in the downlink slot based on the following method.

A. If the uplink time slot that has been allocated exceeds the number of time slots available in the superframe, the AP places an uplink time slot unavailable indicator in the frame body.

B. If the AP does not support any data rate listed in the supported data rates set in the registration request management frame, the AP places an unsupported data rate indicator in the frame body.

C. If there are uplink slots available for allocation and a common data rate supported by both the AP and the STA, the AP allocates one uplink slot and selects an appropriate common data rate according to information such as a carrier/noise ratio within the STA registration request frame, and then transmits a registration response frame to the STA. The frame/message/signal body contains the assigned uplink time slot as well as the selected data rate information. After a successful registration process, the TDF STA and TDF AP negotiate on which uplink timeslot and data rate to use.

Downlink transmission

As described above, the downlink is defined as information transmission from the AP to the STA. The total number of downlink timeslots may change dynamically due to a change in the number of associated STAs throughout the TDF communication process. When the AP is ready to send frames to the associated STA, it compares the time remaining in the remaining downlink slot with the duration required to transmit a particular downlink frame using the negotiated data rate. Then, based on the result, it decides whether the frame should be transmitted at a particular data rate during this TDF superframe. Thus, fragmentation of downlink frames can be avoided in many cases.

Uplink transmission

As described above, the uplink is defined as information transmission from the STA to the AP. Upon receiving the registration response frame from the AP, the STA analyzes the frame body to see if an uplink slot is granted. If not, it pauses and later re-applies for an uplink slot. If granted, the STA begins transmitting uplink traffic during the specified time slot using the data rate indicated in the registration response frame.

At the beginning of an uplink transmission during a designated time slot, if there is at least one outgoing frame in its outgoing queue (buffer), the STA sends the first frame in the queue (buffer) to the AP for transmission. After that, the STA checks the length of the second uplink frame and evaluates whether it is possible to transmit the second/next buffer frame during the remaining duration of the designated slot. If it is not possible to send the next buffered frame, the STA stops the uplink transmission procedure and waits to send the next buffered frame in the assigned timeslot during the next TDF superframe. If it is possible to transmit the next buffered frame during the remaining duration of the specified time slot, the STA immediately transmits the next buffered frame to the destination AP. The transmission process continues in this manner until the end of the designated time slot or until there are no more uplink frames to transmit.

STA and AP equipment

In the network architecture of fig. 1, example modem Stations (STAs) 120 and 140 are depicted as having WLAN RF ports in order to support wireless devices 138 and 158. In one embodiment, the STA may include a WLAN RF interface and a LAN interface port as end user/client interfaces. Further, the STA may have a cable interface to support communication between the AP and the STA and an external client interface port to support a wireless user device or a LAN or WAN interface to the user device. In one embodiment, a STA with a current interface and a client external port may also be referred to as a dual mode device.

Fig. 6a depicts an embodiment of a dual mode STA 600. The STA 600 includes a STA computing device 650 or component, such as a processor, gate array, computing logic, microprocessor, or chipset, or other control function known in the art, that controls communication transactions over the cable interface 610 and the external client interface 620. In this regard, the STA 600 operates in a cable interface/ADoC system mode or in an external interface/client mode. The STA may use a sophisticated WiFi chipset to implement the STA processor/chipset/computing device 650 functionality. Memory 640, such as a solid state disk, a rotating magnetic disk, or an optical disk, may be used to store programs or data information that support STA computing device activity. Additionally, cable interface 610 or client interface 620 may use memory 640 for data access. In support of the present invention, the STA uses a signal strength measurement component 630 to evaluate the received signal strength of transmissions by the AP.

The STA 600 functions to connect with a cable interface to support bi-directional data communication in a cable network using the TDF principle, and an external client interface functions to connect with an antenna or an external network connection to support bi-directional data communication for a client device. STA 600 exchanges data frames between cable interface 610 and external client interface 620, if necessary, to communicate with user/client devices such as PCs, PDAs, routers, switches, printers, smart terminals, etc. in an external network. Data frames are exchanged via a cable interface to the AP in order to access an IP network, such as the internet or an intranet. In the external client interface mode, dual mode device STA 600 operates as an access point for user devices. While in cable interface/ADoC mode, the STA 600 transmits RF energy in the ADoC band (about 1 GHz). In the external client interface mode, a standard wireless band, such as the IEEE802.11 band (approximately 2.4GHz), may be used. In ADoC system/cable interface mode, STAs transmit Medium Access Control (MAC) frames using the TDF protocol based on Time Division Multiple Access (TDMA) method as described above. In the client interface mode, the STA 600 uses any wireless access or wired network protocol, such as the IEEE802.11 protocol or the ethernet protocol.

Fig. 6b depicts one embodiment of an AP 660. The AP interfaces with an external internet protocol source using interface 662. Under the control of the AP computing device 670, the AP has access to content from an IP source and a cable interface 668, the cable interface 668 providing time division multiplexed communication functionality to interact with one or more STAs, such as STA 600. The memory 664 is accessible by the AP computing device 670 and, like memory 640, may be used to store data, programs, and control information in order to properly coordinate the interaction of the STAs on the IP source interface 662 and the cable interface 668. Also cable interface 668 or IP source interface 662 may use memory 664 for data access. The memory may be any form of memory such as a solid state disk, a rotating magnetic disk, or an optical disk. The AP computing device may be any form of computing device that may include a separate or integrated computing device having one or more processors, or computing components such as programmable logic devices, gate arrays, computing logic, microprocessors, or chipsets or other control functions known in the art. The function of the AP computing device is to control the interface of the AP660 and to perform or control any determinations or measurements required by the AP, such as frame loss calculations, signal strength measurement calculations, or any other calculations related to the control of STAs in unicast, multicast, and broadcast modes.

Multicast communication considerations

Multicast transmission is an efficient technique for transmitting the same data to multiple users. Multicasting conserves network resources when transmitting the same data to multiple receivers, as opposed to unicasting to individual users, whereby multicasting is an important service priority in the network, especially for video sharing. However, the IEEE802.11 standard is not directed to multicast communications. The standard indirectly supports multicast transmission by affecting (legacy) broadcasts without any feedback. However, without feedback, the reliability of multicast transmission becomes an issue.

In a wireless environment, multicast transmissions may collide with unicast transmissions when attempting to access the same channel. However, in time division multiplexed systems using IEEE802.11 frames, such as the TDF system described herein, the timeslot allocation mechanism for transmission generally avoids such collisions.

In IEEE802.11 wireless networks, two factors can lead to packet loss; one is channel error and the other is transmission collision. In ADoC systems, the TDF MAC protocol eliminates the packet collision factor. Thus, the main contributor to packet loss is channel error, which is mainly caused by noise and interference. In cable systems, channel errors can be reduced below a desired threshold by selecting an appropriate transmission power, modulation class, and coding scheme.

The data rate adaptation (adaptation) provided by IEEE802.11 is not standardized. Each manufacturer of 802.11 components is free to choose its own data rate adaptation mechanism. Rate adaptation mechanisms for unicast communication are not suitable for multicast due to the lack of feedback signals in multicast, such as MAC layer ACK frames or RTS-CTS message frames. Thus, multicast transmissions generally remain using the lowest modulation level and coding scheme as in broadcast transmissions. The present invention improves this scheme.

As one aspect of the present invention, a novel channel quality feedback mechanism and data rate adaptation technique are established for multicast transmission in a cable environment. With this technique, the data channel can be better utilized and the performance of multicast transmissions can be significantly improved. Such channel data rate adaptation techniques are also helpful to maintain adequate quality of multimedia content transmitted over multicast, and also to achieve throughput gains for unicast transmissions by allocating minimum channel resources on multicast transmissions.

In cable systems, such as the TDF ADoC system, downlink traffic from the AP to the STAs may be transmitted with the strongest power allowed by the system. Nevertheless, uplink transmissions from STAs to APs tend to require careful power control in order to ensure that signals from different STAs to different APs do not interfere with each other. Thus for downlink multicast traffic, no power control mechanism is needed as in the wireless environment, but channel data rate control adaptation is desired in order to enhance transmission rate efficiency and take advantage of the improved channel conditions present in cable networks compared to wireless networks.

The basic steps of the multicast channel adaptation protocol include: (a) periodically performing channel sensing in order to select the STA with the weakest signal strength as a multicast group leader, (b) utilizing a feedback mechanism to the AP, wherein the multicast group leader only acknowledges correct reception of multicast frames, and (c) adjusting the multicast data transmission rate by using a rate reduction mechanism or a rate increase mechanism as conditional grants.

Multicast group leader selection

As an aspect of the invention, the AP uses Signal Strength (SS) as a criterion for selecting the multicast group leader. The leader is the STA in the multicast transmission that experiences the worst quality signal received from the AP in the target multicast group. In a wireless environment, an AP can determine the quality of communication between itself and any station by measuring the signal strength of an uplink frame from an STA, which includes data, RTS-CTS, ACK, and other frames, because the channel between the AP and the STA is considered symmetric. However, there is no guarantee that the uplink and downlink channels in a cable system are symmetric. The asymmetric nature of the cable system is caused by the asymmetric nature of the signal coupler or splitter, such as coupler 115 in fig. 1. In order for the AP to select the correct multicast group leader, the signal strength of the multicast frame from the AP to the STAs must be measured and reported by each STA to the AP.

As an aspect of the invention, the STA periodically measures the signal strength and forwards the signal strength measurements to the AP. Initially, the STAs in the multicast group all respond to the multicast message by sending a measurement of their received signal strength. The AP detects the STA with the lowest received Signal Strength (SS) and designates this STA as the multicast group (mgroup) leader. A more efficient reporting and data rate update scheme can then be used by employing new information elements within the beacon frame.

As an aspect of the present invention, a new management frame information element called a channel sensing information element is added. The information element is a variable length component of the management frame. A general information element has an ID number, a length, and a variable-length data field, as shown in fig. 7 (a). For the channel sensing information element of fig. 7(b), the original reserved element channel sensing ID is used. In the embodiment of fig. 7(b), the length value is thirteen. The variable length data field contains three fields: a multicast group (mgroup) address field (6 bytes), a minimum Signal Strength (SS) value field (1 byte), and a mgroup leader field (6 bytes). The mgroup address field is an address associated with a set of logically related stations by a higher level agreement. In one embodiment, the mgroup address field is a concatenation of the MAC addresses of 01:00:5E and the last 23 bits of class D multicast IP. The minimum SS value field is a signal strength value of the multicast leader determined by a previous (initial) channel sensing process.

The channel sensing element format of fig. 7(b) is available to all STAs in the multicast group by using the beacon frame. In the TDF system, a beacon frame is transmitted as part of a synchronization frame at the start of a superframe as in fig. 5. Thus, in the broadcast beacon frame, all STAs are notified that a particular multicast group contains a particular mgroup leader with a minimum signal strength value.

As an aspect of the present invention, the STA reads a channel sensing element in a beacon frame. When the minimum signal strength value is equal to 0xFF, the STA performs initial channel sensing. This initial channel sensing occurs when the AP has no prior knowledge of the channel conditions of the STAs in a particular multicast group. This value may be presented to all STAs for reading within a beacon frame when the multicast application is initialized and a multicast group is formed. After the initial read, the AP selects the mgroup leader and corresponding signal strength value. This new value is then made available to all STAs within the broadcast beacon frame. When the minimum SS value is between 0x00 and 0xFE, the signal strength value is a value measured through a previous channel sensing process.

Fig. 8 depicts a standard 802.11 management frame with a frame body field. As an aspect of the invention, this frame body field is padded as a beacon frame body with a channel sensing field. The channel sensing field is filled with the channel transmission element of fig. 7 (b). Thus, all STAs that receive the broadcast beacon frame at the beginning of the superframe can determine the multicast group leader and the minimum signal strength value for a particular multicast group.

During the multicast initialization phase, a multicast data stream is transmitted by the AP to the multicast STAs. The AP transmits an initial channel sensing beacon with the minimum signal strength value field populated with 0 xFF. When a channel sensing beacon is received, all STAs in the corresponding multicast group measure a signal strength value of the received beacon. And then, the STA transmits a feedback frame to the AP in order to report the received signal strength. The payload of the feedback frame is the measured signal strength value. Upon receiving the feedback frame, the AP records the minimum signal strength value and selects the STA with the minimum signal strength value as the multicast group leader. The AP loads the identification of the mgroup leader as a reference, the received signal strength of the reference mgroup leader, and the mgroup address into the beacon frame of fig. 8.

After the initial channel sensing, the AP periodically transmits a channel sensing beacon filled with the detected minimum signal strength value in order to require the STA to perform periodic channel measurement. The period during which the channel transmits beacons is selected to enhance adaptivity to changing station conditions. The time period is also selected so as to avoid excessive data rate changes or churning (churning). In operation, if a STA receives a channel sensing beacon whose minimum signal strength value is not equal to 0xFF, the STA first determines whether it belongs to a particular multicast group by reading the mgroup address field. After that, the STA compares the currently measured signal strength value with the minimum signal strength value in the beacon. If the currently measured SS value is less than the minimum SS value, the STA treats itself as a worst-case station for received signal strength and provides the measured SS value to the AP.

If a STA belongs to a particular multicast group and finds that its own Signal Strength (SS) is better than the minimum SS value, the STA determines whether its own MAC address is the same as the MAC address of the mgroup leader. If the two addresses are the same, the STA provides feedback to the AP including the new SS value and its MAC address. The AP modifies the value of the minimum SS value field according to the feedback received from the STA and decides whether to select a new mgroup leader. Fig. 9 shows an example of this processing.

Example STA operation

Fig. 9 depicts an example method 900 of channel sensing processing performed in a STA. At step 910, the STA receives a beacon frame. The signal strength of the received broadcast beacon frame is measured at step 920. At step 930, the minimum signal strength value field is read from the beacon frame. At step 940, a determination is made whether this is initial channel sensing for this multicast group. This determination is made by comparing the minimum signal strength value field of the channel sensing element of the beacon frame with the value as described above. If it is an initial multicast channel sensing, the method 900 moves from step 940 to 980 and sends the measured signal strength value as feedback to the AP. In this case, sending an initial signal strength value measurement to the AP allows the AP to determine the lowest SS value and the corresponding initial mgroup leader.

If it is not initial channel sensing for this multicast group, then step 950 is entered from step 940. At step 950, a determination is made whether the measured value of signal strength is less than the minimum signal strength previously recorded for the mgroup leader. If the measurement is less than the minimum SS value of the mgroup leader, method 900 moves to step 980 where the measurement STA received signal strength is sent back to the AP as feedback. The feedback to the AP includes not only the measured SS value, but also the identity of the responding STA. In this case, at step 950, the responding STA has a measured received signal strength that is less than the mgroup leader previously selected by the AP. As a result, the AP may determine that the responding STA should be a new mgroup leader and that the new SS measurement should be a new minimum SS value for the beacon frame.

If the determination at step 950 is negative, then the measured signal strength of the transmission to the STA is above the minimum SS level. If the measured signal strength is equal to the minimum SS level, the mgroup leader or other station provides no feedback to the AP (not shown in FIG. 9). At step 960, a determination is made as to whether the STA is a mgroup leader. If the STA is the mgroup leader, the method moves to step 980 and reports the newly measured received signal strength measurements back to the AP along with the STA mac address. In this case, feeding back a new higher measured signal strength value to the AP allows the AP to update the minimum SS value for that mgroup leader. In step 960, where the STA is not a mgroup leader, then the method moves to step 970 and no response from the STA to the beacon is provided. In this case, the measured SS is greater than the minimum SS value measured by STAs that are not the mgroup leader (i.e., are not the weakest received signal strength STAs). Thus, no feedback to the AP is required.

Feedback mechanism for multicast services

The AP multicasts the data frame to the STA members in the corresponding multicast group. If the STA is the leader of the multicast group (mgroup) and receives a multicast frame without errors, only the mgroup leader STA sends back an ACK signal/message. The AP then knows that the group leader received the multicast message. If no Negative Acknowledgement (NACK) signals/messages are received from other STAs in mgroup, the AP also assumes that all other STAs within mgroup receive multicast messages without errors. If the mgroup leader STA receives the multicast frame in error, the mgroup leader does nothing. The AP expects the mgroup leader STA to respond to the multicast message only if it is correctly received. If the mgroup leader does not respond, the AP knows that the mgroup leader STA has not correctly received the message.

If the STA is not the mgroup leader and receives a multicast frame without error, the STA does nothing. If the non-mgroup leader STA receives an erroneous multicast message, the non-group leader STA sends a NACK signal/message back.

In summary, after sending a multicast frame, there are many kinds of possible feedbacks. First, if all STAs in the multicast group receive the multicast frame without error, the feedback to the AP is only an ACK signal/message from the multicast group leader. Second, if the mgroup leader receives a frame without an error but at least one other SAT receives a frame with an error, a collision occurs between the ACK signal/message from the mgroup leader and the NACK signal/message from the other STA having a reception error. Third, if the mgroup leader and one or several other STAs receive the multicast message with errors, the feedback is a NACK signal/message or a collision of several NACK signals/messages from the STAs that received the multicast message with errors. Finally, if there is no feedback to the AP after transmission of the multicast message by the AP, the AP concludes that the multicast message/frame is lost. In general, the AP treats all other feedback as NACK collisions, except for the explicit ACK feedback signal from the STA mgroup leader. As a result of the NACK collision, the AP may retransmit the multicast message until it is correctly received, or reduce the data rate and retransmit the multicast message until it is correctly received.

Feedback-based multicast rate selection

The AP initially selects a certain multicast data rate for multicast traffic. When the AP receivesThe multicast transmission rate rises to the next higher level for the next consecutive ACK feedback. In one embodiment of the present invention, the substrate is,is a counter adaptively adjusted in a data rate adaptation process. When the multicast rate is increased, the AP starts a Temporary Higher Rate Period (THRP) during which the AP can decide whether it can use the higher multicast data rate. With respect to a window W at a particular location_dropWhether a frame loss occurs to determine the decision. Will window W_dropIndicating the number of multicast frames transmitted after increasing the data rate for the multicast message/transmission/traffic/data. From frame loss ratio threshold P_{flr_th}A window value is determined. Loss ratio threshold P_{flr_th}Is a probability threshold. When the frame loss rate of the multicast service with higher rate is more than P_{lr_th}The success rate of message transmission at the higher rate is worse than the original lower rate. Thus, the new higher rate should not be used and the AP should reduce its multicast rate back to the original one. P_{flr_th}Is determined as:

where virtual _ transmission _ time means the total transmission time required to transmit a multicast frame, including all PHY and MAC overhead, e.g., PHY preamble, SIFS, DIFS, ACK or NACK and payload (MSDU), and where R_highR for new higher data rates_lowThe previously lower data rate. By P_{flr_th}，W_dropCan be determined asRepresents an integer not less than x):

thereby in the region of W_dropAny missing multicast frame (e.g., a negative acknowledgement "NACK" signal/message event) in a determined THRP causes the AP to reduce its multicast rate back to the original lower one. The above determination for the AP may be made by the AP computing device of fig. 6b or other logic. Frame loss may be indicated on a frame, block, or both basis.

Adaptively adjusting in multicast rate adaptation processingSetting or resetting when a new multicast application is initialized or when the AP starts using a new data rate for multicast trafficIn one embodiment, the reset value is ten. Each AP attempts to increase its multicast rate and the rate is again reduced back to its original lower rate within the THRP time frame. Increasing by a binary backoff (backoff) method

Where T is the number of data rate fallback events experienced by the AP. Increased signal strength of detected multicast groups triggers that the AP will beReset back to ten.

If the AP does not reduce its multicast rate back to its original lower rate during the THRP time frame, the AP uses a higher rate for subsequent multicast data transmissions. During transmission (outside the THRP time period), when in W_dropTwo of the consecutive multicast frames get a NACK event and the multicast transmission rate is reduced to the next lower level.

Example AP operation

Channel sensing operations in an AP are depicted in the example method in fig. 10. Fig. 11 depicts an example method for AP rate adjustment. Considering fig. 10, an example method of performing a channel sensing operation begins at step 1010, where a mgroup address is determined from an IP multicast address. The modified beacon frame is then transmitted to all STAs in step 1015. Since this is the initial setting and the minimum SS value field is 0xFF, then all stations respond by providing their measured signal strength feedback in their agreed TDM slot in step 1020. In step 1025, the AP updates all node entries (entries) for the stations in the multicast group of interest. The AP then determines the multicast group leader as the STA in mgroup with the lowest signal strength indication. The data rate accommodated by this mgroup leader STA is the data rate that the AP initially used to multicast messages to mgroup. Steps 1010 to 1030 are included in an initial period for setting channel sensing according to aspects of the present invention. A repeated and periodic AP operation is started in step 1035.

At step 1035, a beacon frame with the mgroup leader and its corresponding signal strength for that mgroup is sent to all STAs. The multicast message is then sent to the multicast group (mgroup). At step 1040, as a result of the multicast message, the AP determines whether the feedback it receives is from a STA in the mgroup. If no feedback is received, the method 1000 returns to step 1035. The beacon message may be repeated periodically until feedback is received, e.g., a new received signal strength measurement from the STA. If feedback is received, e.g., a new received signal strength indication, a determination is made at step 1050 as to whether the received Signal Strength (SS) value is less than the recorded SS value for the mgroup leader. If the value is smaller, then the minimum SS value is recorded at step 1055. If the mgroup leader has changed, then a new mgroup leader is recorded in step 1055. The method 1000 then returns to step 1035.

At step 1050, if the newly measured SS value from the STA is greater than or equal to the minimum SS value of the mgroup leader, then step 1060 is entered. In step 1060, the AP determines whether the feedback is from the mgroup leader. If the feedback is not from the mgroup leader, step 1060 moves to step 1035. If the feedback is from the mgroup leader, method 1000 moves from step 1060 to step 1065 where the increase in received signal strength and updated values for the mgroup leader are recorded and updated within the beacon frame. At step 1065, since a new minimum signal strength value greater than the previous minimum signal strength value is input, the multicast data rate may be temporarily increased and the method of fig. 11 may be performed in parallel with the method 1000 in order to test for frame loss for the increased data rate. If a new data rate is employed, a new higher data rate is associated with a new minimum signal strength value for the mgroup leader. Step 1065 then returns to step 1035 and updates and periodically transmits beacon frame data.

Fig. 11 depicts a method 1100 for dynamic rate adjustment of multicast messages present in an AP. The method begins at step 1110 where an initial multicast rate is selected for transmitting multicast information frames. Typically, the initial data rate is the data rate associated with the lowest (minimum) received signal strength value of the mgroup leader provided by the AP in the beacon frame. At step 1115, a counter parameter is determinedAnd set it as a reference. In one embodiment of the present invention, the substrate is,is ten. In step 1120, it is performed whether the AP receives the signal from the STA during the normal TDM operationDetermination of one consecutive ACK signal/message. Here, normal TDM operation may include any multicast messages to the multicast group under consideration. Successful successive ACK signals/messages are acquired as a result of successful multicast transmission by the AP to the multicast group of STAs. If notFor a consecutive ACK signal/message, the method 1100 is unsuccessful at the current data rate and the method moves to step 1145 to reduce the data rate. When receivingUpon successful ACK signal/message, the method moves to step 1125 where the transmission data rate is increased for a temporary length of time. The temporary length of time is referred to as a Temporary High Rate Period (THRP). At step 1130, the algorithm is applied to W_dropThe frame loss is recorded. At step 1135, a determination is made as to whether the Temporary High Rate Period (THRP) has expired. If THRP has not expired, process 1100 loops through step 1130. If THRP has expired, the method moves to step 1140.

At step 1140, a determination is made as to whether the frame loss threshold is greater than a threshold value P_{flr_th}And (4) determining. If the frame loss rate is greater than the threshold, the new higher rate does not work well and the method moves to step 1145 where the rate is reduced to its previous lower value. A new one can then be determined and setValue step 1115 restarts the process. If the frame loss threshold does not exceed the threshold, then the new higher rate works well and the method 1100 moves to step 1150 where the temporary higher data rate is employed as the multicast data rate to be used by the AP for a particular mgroup. Processing may then restart at step 1115. Thus, fig. 11 illustrates a technique for dynamically adjusting the transmission data rate of an AP for multicast messages. The above method is adaptive to changes in stations as well as access points. The method determines whether the multicast group can accept the newly selected higher data rate in dependence upon frame loss.

The features and aspects of the described implementations may be applied to various applications. Applications include, for example, individuals using host devices to communicate with the internet at home using an ethernet communications framework over cable as described above. However, the features and aspects described herein may be adapted for use in other application areas, and accordingly other applications are possible and contemplated. For example, the user may be located outside their home, such as in a public place or at their place of employment, for example. Thus, protocols and communication media other than Ethernet and IEEE802.11 may be used. For example, data may be sent and received in the following manner (and using the protocols associated therewith): fiber optic cables, Universal Serial Bus (USB) cables, Small Computer System Interface (SCSI) cables, telephone lines, digital subscriber lines/loop lines (DSL), satellite connections, line-of-sight connections, and cellular connections.

Implementations described herein may be implemented, for example, as a method or process, an apparatus, or a software program. Although described in the context of a single form of implementation (e.g., discussed only as a method), the implementation of the features discussed can also be implemented in other forms (e.g., as an apparatus or program). The apparatus may be implemented on, for example, appropriate hardware, software, and firmware. For example, the methods may be implemented in an apparatus such as, for example, a processor, which refers generally to a processing or computing apparatus, including, for example, a computer, microprocessor, integrated circuit, or programmable logic device. Processing devices may also include communication devices such as, for example, computers, cellular telephones, portable/personal digital assistants ("PDAs"), and other devices that facilitate the communication of information between end-users.

Implementations of the various processes and features described herein may be implemented in a variety of different devices or applications, particularly, for example, devices or applications associated with data transmission and reception. An example apparatus includes: video encoders, video decoders, video codecs, network servers, set-top boxes, laptop personal computers, and other communication devices. It should be clear that the device may be mobile and even mounted on a moving vehicle.

Additionally, the present methods may be implemented by instructions being executed by a processor or other form of computing device, and such instructions may be stored on a computer-readable medium, e.g., an integrated circuit, a software carrier, or other storage device, such as, for example, a hard disk, a compact disk, a random access memory ("RAM"), a read only memory ("ROM"), or any other magnetic, optical, or solid state medium. The instructions may form an application program tangibly embodied on a computer-readable medium, such as any of the media listed above. It should be appreciated that a processor or other form of computing device may include a processor-readable medium as part of a processing unit having, for example, instructions for performing a process.

With respect to cache and storage, it is noted that the various devices throughout the described implementations typically include one or more storage devices or caches. The memory may be, for example, electrical, magnetic, or optical.

As is apparent from the above disclosure, implementations may also produce signals formatted to carry information that may be, for example, stored or transmitted. The information may include, for example, instructions for performing a method, or data generated by one of the described implementations. Such signals may be formatted, for example, as electromagnetic waves (e.g., using the radio frequency portion of the spectrum) or as baseband signals. Formatting may include, for example, encoding a data stream, grouping the encoded stream according to any of various frame structures, and modulating a carrier with the grouped stream. The information carried by the signal may be, for example, analog or digital information. The signal may be transmitted over a variety of different wired or wireless links as is known.

Claims (10)

1. A method of data transmission, comprising:

transmitting a multicast data frame at a first data rate;

receiving a plurality of acknowledgements of successful frame reception using the first data rate;

increasing the first data rate to a second data rate over a period of time in response to the plurality of acknowledgements;

transmitting a multicast data frame at the second data rate;

determining a frame loss during the time period; and

determining a third multicast data rate in response to a multicast frame loss, wherein determining the third multicast data rate in response to the multicast frame loss comprises: using the second data rate as the new multicast data rate if the frame loss is less than a threshold, and wherein the threshold is a probability threshold based on a ratio of transmission times of the second data rate and the first data rate.

2. The method of claim 1, wherein the first data rate is selected by an access point.

3. The method of claim 1, wherein increasing the first data rate to a second data rate further comprises: increasing the first data rate to the second data rate in response to feedback from a multicast group.

4. The method of claim 3, wherein the feedback from the multicast group comprises: an increased received signal strength indication of a reference station.

5. The method of claim 4, wherein the increased received signal strength indication is transmitted to all stations in the multicast group using a beacon frame.

6. The method of claim 4, wherein the reference station is characterized by a lowest received signal strength value within the multicast group.

7. The method of claim 3, wherein the feedback from the multicast group comprises: receiving a plurality of consecutive reply messages, each reply message received after transmission of a multicast data frame to the multicast group.

8. The method of claim 7, wherein the number of consecutive reply messages exceeds a threshold and the threshold is adaptively adjusted.

9. The method of claim 7, wherein only reference stations in the multicast group send acknowledgement messages upon successful reception of a multicast data frame and wherein other stations in the multicast group other than the reference stations transmit negative acknowledgement messages upon reception of an erroneous multicast data frame.

10. The method of claim 1, wherein determining a third multicast data rate in response to the multicast frame loss comprises: the second data rate is reduced back to the first data rate if the frame loss reaches a threshold.